Put it this way. On my old amd athlon 2800+ I didnt clean the fan and heat sink for about 4 months.
I was getting idle temps of 55-60 degrees C.
I cleaned it and it drop to 35-40 at idle.
Your best bet is make sure you clean it completely once a month. PSU is a big attractor of dust, as is HSF for the cpu, the capacitors near the northbridge, the northbride itself.
Inbetween all expansion slots, HDD, Optical drives. and all the air intakes and outlets.
A clean system is a healthy system.
AMD Athlon 64 3200+ @ 2.4Ghz \"Winchester\" 939 pin
Asus A8V Deluxe rev 2.0 Bios 1010.003
2x 512 3200 Corsair Twin XMS Dual Channel
120GB 7200rpm Maxtor Diamondmax Plus 9 2MB cache
160GB 7200rpm Seagate Barracuda SATA 8MB Cache Configured for RAID 0+1
Chaintec Geforce 6800 @ 411Mhz Core, 877 Mhz memory
Soundblaster Audigy 2 ZS Platinum
MSI DVD ROM
17inch Samsung monitor